you are want calc robux.
The widget below can be used to show the exchange rate between Robux and USD, and vice versa.

DevEx rate: Rate at which Robux can be exchanged into USD through the DevEx program.
Purchase rate: Rate at which Robux can be purchased via USD.

Formula for DevEx rate (Robux -> USD): u=0.0035r with u being USD and r being Robux.
Formula for Purchase rate (Robux -> USD): u=0.0125r with u being USD and r being Robux.
This is a free and amazing tool, it's a powerful robux and usd calculate for ROBLOX.
This app very is simple
Please Note:
* "Free Robux Calc Free" is only to help gamer in calculating cost of the robux
This app is NOT a cheat or hack to get free robux in game